In-situ real-time monitoring technique and apparatus for endpoint detection of thin films during chemical/mechanical polishing planarization

ABSTRACT

A technique and apparatus is disclosed for the optical monitoring and measurement of a thin film (or small region on a surface) undergoing thickness and other changes while it is rotating. An optical signal is routed from the monitored area through the axis of rotation and decoupled from the monitored rotating area. The signal can then be analyzed to determine an endpoint to the planarization process. The invention utilizes interferometric and spectrophotometric optical measurement techniques for the in situ, real-time endpoint control of chemical-mechanical polishing planarization in the fabrication of semiconductor or various optical devices. The apparatus utilizes a bifurcated fiber optic cable to monitor changes on the surface of the thin film.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of and claims priority to U.S.application Ser. No. 07/996,817, filed on Dec. 28, 1992, the entirety ofwhich is incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention is directed to a technique and apparatusfor the optical monitoring and measurement of a surface undergoingrotation, particularly for in situ, real-time monitoring of any thinfilm undergoing rotation and simultaneous dimensional changes. It isparticularly useful in the field of wafer planarization for producingwafers of extreme flatness and uniformity that are desirable in theproduction of semiconductor and integrated circuits.

BACKGROUND OF THE INVENTION

[0003] As microelectronic device dimensions continue to shrink,patterning problems increasingly hinder integrated circuit andsemiconductor device fabrication. Semiconductor device fabrication oftenrequires extremely planar surfaces and thin films of precisethicknesses. The surfaces requiring planarization and thickness controlin semiconductor devices include areas or layers of dielectric material(such as SiO2) on the surface of semiconducting materials and otherdevice pattern layers. The insulating dielectric layers and other devicelayers need to be extremely planar because irregularities and roughtopography lead to fabrication problems, including Depth of Focus budget(hereafter DOF) problems. Since an irregularity in the surface can causepart of the surface to be out of focus at a particular distance betweenthe optical system and the wafer, errors in pattern formations canoccur. Also, the thickness of layers needs to be precisely controlledbecause variations in thickness may affect the electrical properties ofthe layers and adjacent device patterns, particularly in theinterconnections between the different layers of microelectronicdevices.

[0004] The precise control of layer thickness is also crucial insemiconductor device fabrication. In VLSI technology, certain layers ofmulti-layer devices are generally electrically interconnected. Theselayers are also typically insulated from various levels by thin layersof insulating material such as SiO.sub.2. In order to interconnect thedevice layers, contact holes are often formed in the insulating layersto provide electrical access therebetween. If the insulating layer istoo thick, the layers may not connect, if the layer is too thin, thehole formation process may damage the underlying device layer.

[0005] Due to the various inadequacies of other planarization methods(such as spin-on-glass and etchback), Chemical/Mechanical Polishing(hereafter CMP) planarization machines and other lapping machines havebeen developed and employed to provide planar surfaces for semiconductordevice fabrication. Generally, CMP is a technique of planarizing wafersthrough the use of a polishing pad attached to a rotating table. Thewafer is held by a chuck above a polishing pad which rotates on aspindle. The wafer is pressed downward against the polishing pad. Thenap of the rotating pad removes bits of the film layer, therebyplanarizing the surface. A solution of pH controlled fluid and colloidalsilica particles called slurry flows between the pad and the wafer toremove material from the polishing area and to enhance the planarizationof the wafer surface.

[0006] A typical method of determining the endpoint of CMP and lappingmachines is to measure the amount of time needed to planarize standardwafer(s), and then to process the remaining wafers for a similar amountof time. In practice, this process is very inefficient because it isvery difficult to determine the precise rate of film removal, aspolishing conditions and the polishing pad change from wafer to waferover time. As a result, it is often necessary to inspect the wafersindividually after planarization, which is time-consuming and expensive.Thus, the CMP process could be significantly improved by introducing anin situ, real-time measurement and control system.

[0007] The ability to monitor and control the CMP process has beendirectly and indirectly addressed by several techniques. One method isbased on measuring capacitance (U.S. Pat. No. 5,081,421). The theorybehind this method is that the electrical capacitance of the waferchanges as the wafer surface is planarized. The two primary drawbacks ofthe method are its limited accuracy and its pattern dependency. Itsaccuracy can be compromised by the patterns of underlying layers whichmay also affect the capacitance of the entire system.

[0008] One direct method has been proposed which uses laser light tomake interferometric readings on the polished side (front side) of asection of the wafer which overhangs the polishing pad (U.S. Pat. No.5,081,796). The disadvantages of this system are that it requiressubstantial modification of the conventional CMP process since part ofthe wafer must overhang the edge of the polishing pad, leading topolishing uniformity problems, and also, the monitored spot on therotating wafer must be coordinated with the apparatus underneath thewafer overhang.

[0009] An indirect method of monitoring CMP has been developed whichsenses the change in friction between the wafer and the polishingsurface (U.S. Pat. No. 5,036,015). The change in friction may beproduced when, for instance, an oxide coating of the wafer is removedand a harder material is contacted by the polishing pad. The accuracy ofthe method suffers from variations in changing pad conditions. Inaddition, use of the method may be limited by the need to sense thefriction generated by different materials.

[0010] Another indirect method of monitoring CMP has been developedutilizing the conductivity of the slurry during polishing (U.S. Pat. No.4,793,895). When metal layers are exposed during the CMP process, theelectrical resistivity of the slurry and wafer changes due to theexposed metal on the wafer surface. The obvious drawback of this methodis that it requires having exposed metal surfaces for monitoring. Thisis not possible for most types of polishing situations.

[0011] Another indirect method of monitoring CMP has been developedutilizing the waste slurry off the pad during planarization (U.S. Pat.No. 4,879,258). Certain materials are imbedded in the dielectric layerand monitored as they are planarized and fall off the polishing pad. Theobvious drawbacks to this method include the time delay betweenplanarization and when the slurry reaches the edge of the pad (estimatedto be approximately 30 seconds) and also the low levels of sensitivityand signal noise introduced by the materials left over from the previouswafers. This method is not an active, real-time method.

[0012] These and other endpoint detection techniques do not offereffective and accurate control of the CMP process in an in situ,real-time manner. These schemes either compromise the accuracy of theendpoint detection and/or require significant alterations of the CMPprocess.

SUMMARY OF THE INVENTION

[0013] An object of the present invention is to avoid theabove-mentioned problems by allowing for the monitoring of a region of afilm on a substrate undergoing thickness changes, thus enabling endpointdetection in an in situ, real-time manner. In addition, if the monitoredregion is sufficiently small, a spot on the wafer can be dedicated forendpoint purposes. The dedicated endpoint spot can remove signalproblems associated with the layer's topology, patterns, and multiplefilm layers.

[0014] The present invention thus provides an apparatus and method forthe optical illumination and monitoring of a section on a thin filmlayer undergoing dimensional changes. Light from a light source istransmitted to a monitoring area on the layer, preferably through theback side of the substrate, and relayed back to an analyzer whichevaluates changes in thickness of the substrate based oninterferometric, spectrophotometric, and/or absorption changes. In apreferred embodiment, the light signal is advantageously measured fromthe back side of the substrate, which facilitates implementation sincethe monitored region of the wafer and the monitoring apparatus do notneed to be timed and coordinated during the process.

BRIEF DESCRIPTION OF THE FIGURES

[0015]FIG. 1 is a side view of a representative semiconductor devicewith a device pattern on a substrate of semiconductive material and athick, unplanarized dielectric layer over the pattern and substrate.

[0016]FIG. 2 illustrates the semiconductor device of FIG. 1 after thedielectric layer has undergone CMP planarization.

[0017]FIG. 3 is a side view of a rotating coupler fitted withfiber-optic cable.

[0018]FIG. 4 is a side view representation of the fiber-optic embodimentof the present invention integrated with a CMP assembly. The fiber-opticapparatus is pictured in both a front side and a back side approach.

[0019]FIG. 5 shows another embodiment of the present invention utilizinga light source which transmits light in the direction of the wafer,where it is reflected off the wafer's surface and the reflection ismonitored by a photodetector which converts the light signal to anelectrical signal. The electrical signal can be relayed to an analyzerafter passing through an electrical slip ring.

[0020]FIG. 6 illustrates a wafer-holding chuck shown in FIG. 4, whereinair has been pumped into a cavity above the wafer to compensate for lossof pressure against the back of the wafer where holes are located.

DETAILED DESCRIPTION OF THE INVENTION

[0021] In one embodiment, the present invention is a method ofmonitoring the thickness change of a film on a substrate comprisingilluminating a section of the film through the back side (the side whichis not being processed) of the substrate, measuring a light signalreturning from the illuminated section, and determining change inthickness of the film on a substrate based on the measured light signal.Thickness change can be determined by an analyzer, which analyzesinterferometric, spectrophotometric, absorption, and/or other opticalchanges in the measured light signal. optionally, if the substrate isundergoing rotation, the method further comprises the step of relayingthe light signal to a rotating coupler which links the signal to ananalyzer.

[0022] Another embodiment of the present invention is a method ofmonitoring the change in thickness of a film on a substrate comprisingilluminating a section of the film from the front side (i.e., the sidebeing polished) of the substrate, measuring a light signal returningfrom the illuminated section, and relaying the light signal to arotating coupler which connects to an analyzer, and monitoring thicknesschange with the analyzer, which analyzes interferometric,spectrophotometric, absorption, and/or other optical changes in themeasured light signal.

[0023] Another embodiment of the present invention is an apparatus formonitoring thickness change in a film on a substrate comprising a lightsource which illuminates a section of the film from either the frontside or back side of the substrate to generate a light signal, means fordetecting the light signal, means for analyzing the light signal, arotating coupler for relaying the light signal from the illuminatedsection to the means for analyzing the light signal, and optionally oneor more focusing lenses.

[0024] Preferably, the apparatus comprises

[0025] (i) a bifurcated fiber-optic cable having a common leg and twobifurcated legs,

[0026] (ii) a rotating fiber-optic cable with two ends,

[0027] (iii) a light source,

[0028] (iv) means for analyzing a light signal, and

[0029] (v) a rotating coupler having a stationary end and a rotatingend,

[0030] wherein the first bifurcated leg of the bifurcated fiber-opticcable is connected to the light source, the second bifurcated leg isconnected to the means for analyzing a light signal, and the common legis connected to the stationary end of the rotating coupler,

[0031] and wherein one end of the rotating fiber-optic cable isconnected to the rotating end of the rotating coupler and the other endis held in close proximity to the film on a substrate undergoingprocessing.

[0032] “Close proximity” includes any distance between the end of therotating fiber-optic cable and the film on the substrate that is closeenough to permit effective illumination of the monitored area of thefilm and effective reception of the returning light signal. A preferreddistance is less than or equal to about 1 cm.

[0033] The rotating fiber-optic cable serves both to transmit light fromthe light source to the illuminated section and to transmit thereturning light signal on its way back from the illuminated section.Light from the light source travels down the first bifurcated leg of thebifurcated fiber-optic cable and passes through the rotating couplerdown the rotating fiber-optic cable to illuminate the film on asubstrate. The returning light signal is relayed by the secondbifurcated leg of the bifurcated fiber-optic cable to the analyzer. Inaddition, more than one section of the film on a substrate can bemonitored at a time by using multiple legs of rotating fiber-opticcables which pass through one or more rotating couplers.

[0034] Preferably, the fiber-optic cable comprises multiple optic fibersbundled together. However, the fiber-optic cable may comprise a singlefiber. Alternatively, it may be a combination of bundled fiber-opticcable and single fiber-optic cable. Alternatively, it may be multiplefiber-optic cables bundled together.

[0035] The term “substrate” includes any insulating, conductive, andsemiconductive materials known in the art. Preferred substrates arewafers such as silicon wafers, gallium-arsenide wafers, and silicon oninsulator (SOI) wafers.

[0036] The term “film on a substrate” includes various dielectric layersknown in the art, such as SiO.sub.2, metal layers such as tungsten andaluminum, and various other films such as silicon films found on thesubstrate as defined above. The films also include resist layers.

[0037] The film undergoing thickness change, for example, may be a filmon a substrate in a chemical mechanical polishing (CMP) process, achemical vapor deposition process, a resist development process, apost-exposure bake, a spin coating process, or a plasma etching process.In the CMP embodiment, the film is located at the interface of thesubstrate and the polishing pad.

[0038] The term “light source” includes any source of light capable ofilluminating a film on a substrate in the range of about 200 to about11,000 nanometers in wavelength. If the light signal is measured fromthe back side of the substrate, the wavelength is preferably betweenabout 1,000 and about 11,000 nanometers. A preferred back sidewavelength is 1,300 nanometers. If the light signal is measured from thefront side, then the wavelength is preferably between about 200 andabout 11,000 nanometers. A preferred front side wavelength is 632.8nanometers. Preferably, the section of the film on the substrate iscontinuously illuminated by the light source, although illumination canbe at timed intervals.

[0039] Suitable means for analyzing the light signal, or “analyzers”,include photodetectors, interferometers, spectrophotometers, and otherdevices known in the art for measuring interferometric,spectrophotometric, absorption, and/or other optical changes.

[0040] Suitable rotating couplers include any couplers known in the artfor coupling a rotating member to a non-rotating member provided thatlight is permitted to pass between the ends of the two members. Suchcouplers are disclosed, for example, in U.S. Pat. Nos. 4,398,791 and4,436,367. Preferably, the means for coupling the rotating fiber-opticcable to the bifurcated fiber-optic cable which is not rotatingcomprises a rotating member which attaches to one end of the rotatingfiber-optic cable. The rotating member is fitted into a stationarymember of the rotating coupler which is attached to the common leg ofthe bifurcated fiber-optic cable. The coupler is designed such that theend of the rotating fiber-optic cable is held in close proximity,preferably less than 1 cm, to the common leg of the bifurcatedfiber-optic cable, thereby permitting light to pass between the twoends. Optionally, the cable ends can be fitted with focusing lenses toenhance signal transmission.

[0041] The rotating coupler can be replaced with other types ofcouplers, including off-axis fiber-optic couplers, electrical sliprings, or a combination of the aforementioned couplers, or with othermeans of signal rotation decoupling.

[0042] Typical CMP machines in which the methods and apparatus of thepresent invention can be used are those produced by R. HowardStrasbaugh, Inc. in San Luis Obispo, Calif.; Westech Systems, Inc. inPhoenix, Ariz.; and Cybeq Systems in Menlo Park, Calif.

[0043]FIGS. 1 and 2 illustrate CMP planarization of a semiconductordevice wafer. In FIG. 1 is shown a representative semiconductor device,which includes a dielectric layer such as SiO2, 1, deposited on thesurface of a device pattern, 2, formed on a silicon wafer substrate, 3.The dielectric layer may be formed in a manner such as chemical vapordeposition (CVD) of oxide, spin-on-glass, or by other means. FIG. 2shows the wafer of FIG. 1, but with the dielectric layer, 1, planarizedto a preselected thickness after CMP. The device pattern, 2, and thewafer substrate, 3, are relatively unchanged in this process.

[0044]FIG. 3 is a side view representation of a preferred embodiment ofthe optical rotating coupler apparatus. FIG. 3 shows a bifurcatedfiber-optic cable, 4, one bifurcated leg of which connects to a lightsource, 7, and another bifurcated leg of which connects to aphotodetector, 8, which in turn is connected to signal conditioningelectronics and a computer processing and control system (not shown).The common leg of the bifurcated fiber-optic cable, 4, connects to anoptical rotating coupler, 5. A rotating fiber-optic cable, 6, extendsfrom the rotating coupler to the area of the wafer substrate to bemonitored. The fiber-optic cables, 4 and 6, may be either single fiberor bundled fiber types. Also, it is possible to use several fiber-opticcables or fibers instead of one cable or fiber. Also, it is possible tomake a hybrid single fiber and bundled fiber cable embodiment, e.g.,cable 4 is single fiber and cable 6 is bundled cable. Focusing lensesare not necessary at the monitoring end of cable 6 if the cable is fixedsecurely and closely enough to the monitoring area of the wafersubstrate. Preferably, the distance between the end of cable 6 and thewafer substrate is less than I cm.

[0045]FIG. 4 is a side view representation of a typical CMP planarizeror lapping machine adapted with the apparatus shown in FIG. 3. Theapparatus may be set up in the planarizer from a back side approach with4, 5, and 6, or from a front side approach with 4′, 5′, and 6′. Only oneof the approaches, either back side or front side, is needed at any onetime for effective monitoring. In FIG. 4, the wafer holding-chuck andspindle, 12, is shown integrated with a rotating coupler, 5, for theback side approach. The bifurcated fiber-optic cable, 4, is fed into thespindle, 12, and connected to the stationary portion of the rotatingcoupler, 5, as shown in FIG. 3. The rotating fiber-optic cable, 6, isfed down the spindle to the monitored area of the wafer, 11, whichmonitored area is optionally a patternless area such as a clear area ordie or which is optionally an area having a pattern. The wafer, 11 isheld to the chuck by a mounting pad or “fixture” which is generallyattached to the chuck by a chemical adhesive. The fixture is oftencomposed of a base matrix held together by a polyurethane surface layer.The outer surface which holds and presses against the back of the wafer,grips the wafer, and also provides uniform support for the wafer.

[0046] In the other embodiment shown in FIG. 4, the rotating table base,10, and platen, 9, which holds the polishing pad is shown integratedwith a rotating coupler, 5′, for a front side approach to the wafer. Thebifurcated fiber-optic cable, 4′, is fed into the rotating table base,10, and connected to the stationary portion of the rotating coupler, 5′.The rotating fiber-optic cable, 6′, connected to the rotating shaft ofthe rotating coupler, 5′, is adjacent to the monitoring area of thewafer. As the polishing pad attached to the platen, 9, is generallyperforated, the end of the fiber-optic cable, 6′, can be embedded in oneof the holes. Translucent slurry solution flows in between the polishingpad and the wafer, scattering most visible light wavelengths.Optionally, signal enhancement means can be used to compensate forslurry scattering effects of different light wavelengths. In thepreferred embodiment, the source light for the front side method is632.8 nanometer wavelength light, a wavelength balancing the concerns oflight signal transmission through the slurry, and also accuracy ofmeasurement. The rotating fiber-optic cable, 6′, embedded in thepolishing pad must transmit and receive the interference signal when itis positioned over the measurement area on the wafer, 11. This iscoordinated so that the monitoring optics pass over the monitored areaon the wafer using ordinary skill in the art.

[0047]FIG. 5 illustrates the use of an electrical slip ring assembly. Alight source, 14, transmits light to a point on the wafer, 11, whichcauses the light to reflect in the direction of the photodetector orother light-monitoring electronics, 13, which convert the light signalinto an electrical signal. The electrical signal may then be passed onto an analyzer, 12, and finally, to an electrical slip ring, 15, whereit is decoupled from rotation and passed on to other analyzers whichmonitor the progress of CMP.

[0048]FIG. 6 illustrates a wafer-holding chuck and spindle, 12, whereinthe rotating fiber-optic cable, 6, has been routed from the coupler, 5,through the wafer-holding chuck, 12, to a point behind the wafer, 11,during the CMP process. If the pad or “fixture” which holds the waferdoes not allow the passage of light, then it can be perforated toprovide an optical access to the wafer. In order to compensate for theloss of pressure against the wafer at the point of optical access, airmay optionally be pumped into the cavity to press against the wafer andcompensate for the loss of pressure. Alternatively, if the fixture isable to transmit light, then the perforation for optical access is notnecessary.

[0049] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope of the invention.

I claim:
 1. An apparatus for chemical mechanical polishing of a wafer,comprising: (a) a platen supporting a polishing surface; (b) a chuck tohold the wafer against the polishing surface; (c) a motor coupled to atleast one of the polishing surface and the chuck to generate relativemotion therebetween; and (c) an endpoint detector, comprising (c1) alaser interferometer to generate a laser beam that is directed towardsthe wafer and to detect light reflected from the wafer, and (c2) a holeformed in the platen through which the laser beam passes to reflect offa section of the wafer when the hole is positioned adjacent the sectionof the wafer.
 2. The apparatus of claim 1, wherein the hole is filledwith a portion of a fiber-optic cable.
 3. A chemical mechanicalpolisher, comprising: a polishing surface that is movable relative to asubstrate; at least one light source to transmit light through thepolishing surface to a film on the substrate; and at least one devicethat detects interferometric change in reflected light generated whenlight is transmitted through the polishing surface to the film.
 4. Thechemical mechanical polisher of claim 3, wherein the at least one devicecomprises a detector to detect said interferometric change and ananalyzer for controlling the chemical mechanical polisher in response tothe detected interferometric change.
 5. The chemical mechanical polisherof claim 4, wherein the analyzer analyzes interferometric change in thereflected light to determine a change in dimension of the film.
 6. Thechemical mechanical polisher of claim 5, wherein the analyzer analyzesinterferometric change in the reflected light using interferometry atone wavelength.
 7. The chemical mechanical polisher of claim 5, whereinthe analyzer analyzes interferometric change in the reflected lightusing spectrophotometry over a continuous range of wavelengths.
 8. Thechemical mechanical polisher of claim 5, wherein the analyzer analyzesinterferometric change in the reflected light to determine a change inthickness or planarity of the film.
 9. The chemical mechanical polisherof claim 3, wherein incident and reflected light are transmitted througha rotating fiber optic cable embedded in a rotating platen below thepolishing pad.
 10. The chemical mechanical polisher of claim 3, whereinincident light is transmitted to a section of the film.
 11. The chemicalmechanical polisher of claim 3, wherein incident light is transmitted tomore than one section of the film.
 12. The chemical mechanical polisherof claim 3, wherein the light source produces a light of at least onewavelength between 200 and 11,000 nanometers.
 13. The chemicalmechanical polisher of claim 3, wherein the light source produces laserlight.
 14. A method of chemical mechanical polishing, comprising:holding a substrate against a polishing surface; moving the polishingsurface relative to the substrate to polish a film on the substrate;illuminating at least one section of the film with light transmittedthrough the moving polishing surface during polishing of said at leastone section; and detecting interferometric change in light reflectedfrom the at least one illuminated section of the film that passes backthrough the polishing surface.
 15. The method of claim 14, wherein theilluminating step includes generating a light beam from at least onelight source that illuminates the at least one section of the film andthe detecting step includes detecting a reflected portion of the lightbeam with at least one device that detects the interferometric change.16. The method of claim 15, wherein said interferometric change isdetected when said at least one section of the film passes over said atleast one device.
 17. The method of claim 15, wherein light from thelight source that illuminates said at least one section and reflectedlight pass through a fiber optic cable embedded in the polishingsurface.
 18. The method of claim 18, further comprising controllingthickness change in the film in response to the detected interferometricchange.
 19. The method of claim 15, wherein the light directed throughthe polishing pad to the at least one section of the film comprises atleast one wavelength between 200 and 11,000 nanometers, and theinterferometric change in the reflected light is analyzed over one ormore wavelengths.
 20. The method of claim 14, wherein more than onesection of the film is illuminated.
 21. The method of claim 14, whereinpolishing the film comprises reducing the thickness of the film orplanarizing the film.
 22. The method of claim 14, wherein a polishingendpoint is detected based on said interferometric change in thereflected light.
 23. A method of claim 22, wherein the film is a metalfilm.
 24. The method of claim 14, wherein the film is formed over asubstrate.
 25. The method of claim 24, wherein the substrate comprisesat least one of an insulating material, a conductive material, asemiconductive material, a silicon wafer, a gallium arsenide wafer and asilicon on insulator.
 26. The method of claim 24, wherein the substratecomprises a semiconductor device over a silicon wafer.
 27. The method ofclaim 14, wherein the film comprises at least one of an SiO₂ layer, aspin-on-glass layer, a tungsten layer, an aluminum layer, a siliconlayer and a photoresist layer.
 28. The method of claim 14, wherein thefilm comprises a dielectric layer over a semiconductor substrate. 29.The method of claim 14, wherein the film comprises at least onedielectric layer over at least one metal layer.
 30. The method of claim14, wherein the film comprises a part of a semiconductor device or anintegrated circuit.
 31. The method of claim 14, wherein said at leastone section of the film is illuminated with light including at least onewavelength between 200 and 11,000 nanometers.
 32. A method of making aplanarized substrate comprising polishing a film on a substrate with amoving polishing pad; illuminating at least one section of the film withlight transmitted through the moving polishing pad during polishing ofsaid at least one section; and detecting interferometric change in lightreflected from the at least one illuminated section of the film.
 33. Achemical mechanical polisher, comprising: a polishing material having atleast one optical access through which light can be transmitted to aportion of a film on a substrate; a platen to support the polishingmaterial; and an interferometer to direct a light beam through thepolishing material and detect interferometric change in reflected light.34. The chemical mechanical polisher of claim 33, wherein the at leastone optical access in the polishing pad is transmissive to lightcomprising at least one wavelength between 200 and 11,000 nanometers.35. The chemical mechanical polisher of claim 33, wherein the at leastone optical access is a portion of a fiber optic cable.
 36. The chemicalmechanical polisher of claim 33, further comprising a focusing lens toenhance transmission of light passing between the polishing material andthe film on the substrate.